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Articles

Photochemical Characteristics in a Soybean Mutant ¹

^a Charles F. Kettering Research Laboratory, Yellow Springs, Ohio 45387

Chloroplasts were isolated from wild type (DG) and heterozygous mutant (LG) soybean (Glycine max) leaves, and various biochemical functions were compared. Noncyclic electron transport, and its coupled phosphorylation, cyclic phosphorylation and H⁺ ion transport in both systems, were 3 to 5 times faster in rate (on a chlorophyll basis) in the mutant plastids. On a chloroplast lamellar protein basis, the mutant plastid rates were 1.5 to 2.5 times the wild type rates.

Plastoquinone (PQ) reduction and oxidation (rates and extent) were measured by following absorbance changes at 260 nanometers with the repetitive flash technique. Mutant plastids have about a 2-fold greater apparent first order rate constant for PQ oxidation and a 3- to 5-fold larger pool of rapidly reducible PQ. Plastoquinone oxidation has been identified by other workers as the rate-limiting step in electron transport. Assuming the PQ oxidation is a first order process (d(PQH₂)/dt = k_D[PQH₂]t), the observed increase in k_d for the LG (k_dLG 2k_dDG) and the greater steady state amount of rapidly turning over PQ, [PQH₂]_LG>[PQH₂]_DG, could account for the 3- to 5-fold greater rates of electron transport and phosphorylation found in the mutant chloroplasts.

Light saturation for noncyclic photophosphorylation and photosystem 2 plus 1 electron transport occurred at similar intensities for both LG and DG plastids. Relative quantum requirements extrapolated to zero intensity were similar in the LG and DG, although at finite light intensities the LG had a better relative quantum efficiency.

Ammonium chloride concentrations needed to inhibit cyclic photophosphorylation 50% were similar in both LG and DG plastids. Nigericin, poly-L-lysine, and chlorotri-n-butyltin, were needed in concentrations 5 to 10 times greater in the LG to yield 50% inhibition at comparable chlorophyll concentrations.